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Register a new userElectrically charged Andreev modes in two-dimensional tilted Dirac cone systems
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In a graphene-based Josephson junction, the Andreev reflection can become specular which gives rise to propagating Andreev modes. These propagating Andreev modes are essentially charge neutral and therefore they transfer energy but not electric charge. One main result of this work is that when the Dirac theory of graphene is deformed into a tilted Dirac cone, the breaking of charge conjugation symmetry of the Dirac equation renders the resulting Andreev modes electrically charged. We calculate an otherwise zero charge conductance arising solely from the tilt parameters $veczeta=(zeta_x,zeta_y)$. The distinguishing feature of such a form of charge transport from the charge transport caused by normal electrons is their dependence on the phase difference $phi$ of the two superconductors which can be experimentally extracted by employing a flux bias. Another result concerns the enhancement of Josephson current in a regime where instead of propagating Andreev modes, localized Andreev levels are formed. In this regime, we find enhancement by orders of magnitude of the Josephson current when the tilt parameter is brought closer and closer to $zeta=1$ limit. We elucidate that, the enhancement is due to a combination of two effects: (i) enhancement of number of transmission channels by flattening of the band upon tilting to $zetaapprox 1$, and (ii) a non-trivial dependence on the angle $theta$ of the the tilt vector $veczeta$.
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Hyperbolic materials are receiving significant attention due to their ability to support electromagnetic fields with arbitrarily high momenta and, hence, to achieve very strong light confinement. Here, based on first-principles calculations and many-body perturbation theory, we explore the characteristic of two-dimensional plasmon modes and its hyperbolic properties for two phases of single layer boron hosting tilted Dirac cone, namely, the $hr$-$sB$ and $8Pmmn$ borophene. In-plane anisotropy in borophene is manifested in the structural, electronic, vibrational and optical properties. We find two hyperbolic regimes for both phases of borophene, where the high-energy one is located in the visible range. The $hr$-$sB$ borophene is characterised with an intrinsic high carrier density and it supports strong hyperbolic plasmon modes in the visible part of the spectrum. The $8Pmmn$ borophene, on the other hand, resembles the prototypical Dirac material graphene, and upon carrier doping acquires anisotropic Dirac plasmons in the mid-infrared. We have also investigated the impact of the electron-phonon coupling and Landau damping on these hyperbolic plasmon modes. Our results show that borophene, having high anisotropy, intrinsic high carrier concentration, low-loss hyperbolic Dirac plasmon modes, and high confinement can represent a promising candidate for low-loss broad band surface plasmon polariton devices.
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The electronic structure of FeSe thin films grown on SrTiO3 substrate is studied by angle-resolved photoemission spectroscopy (ARPES). We reveal the existence of Dirac cone band dispersions in FeSe thin films thicker than 1 Unit Cell below the nematic transition temperature, whose apex are located -10 meV below Fermi energy. The evolution of Dirac cone electronic structure for FeSe thin films as function of temperature, thickness and cobalt doping is systematically studied. The Dirac cones are found to be coexisted with the nematicity in FeSe, disappear when nematicity is suppressed. Our results provide some indication that the spin degrees of freedom may play some kind of role in the nematicity of FeSe.
Tilted Dirac/Weyl fermions admit a geometric description in terms of an effective spacetime metric. Using this metric, we formulate the hydrodynamics theory for tilted Dirac/Weyl materials in $d+1$ spacetime dimensions. We find that the mingling of spacetime through the off-diagonal components of the metric gives rise to: (i) heat and electric currents proportional to the {em temporal} gradient of temperature, $partial_t T$ and (ii) a non-zero Hall condductance $sigma^{ij}propto zeta^izeta^i$ where $zeta^j$ parametrizes the tilt in $j$th space direction. The finding (i) above suggests that naturally available sources of $partial_t T$ in hot deserts can serve as new concept for the extraction of electricity from the spacetime geometry. We find a further tilt-induced non-Drude contribution to conductivity which can be experimentally disentangles from the usual Drude pole.
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